Stents, such as braided or knitted stents for surgical implantation in body lumens (tubular vessels), are known for repairing or strengthening the vessels. A stent essentially is a hollow tube that supplements the body lumen. With respect to the medical condition of stenosis, in which a body lumen tends to collapse or otherwise close, the stent supports the wall of the vessel to prevent it from collapsing or closing. A blood vessel that is narrowed due to the build up of intra-vascular plaque is one example of a stenosis. With respect to the medical condition of aneurism, in which a body lumen is weakened and cannot properly withstand the internal pressure within the vessel and bulges out or ruptures, a graft or stent-graft serves essentially the opposite function in that it substitutes for or supplements a weakened portion of the vessel. Stents are known for insertion in blood vessels, bile ducts, colons, trachea, esophagi, urethra, ureters, nasal passages, ductal systems, etc.
Stents are known that are fabricated from rigid, but flexible materials that, when bent by force, tend to retain the bent shape. Such stents may be inserted into the body lumen in an unstressed, radially minimal shape while mounted over a deflated balloon. When the stent is in situ, the balloon is inflated in order to radially expand the stent, which will then retain the radially expanded shape after the balloon is deflated and removed.
Another type of stent is termed a self-expanding stent. Self-expanding stents can be compressed radially, but will expand to their original shape once the radially constrictive force is removed. Some types of self-expanding stent are formed from materials that are superelastic or have shape memory characteristics. Such stents are commonly made of Nitinol, a biocompatible alloy that, depending on its chemical composition and thermomechanical history, may be either a shape memory material or a superelastic material. The ULTRAFLEX stent manufactured and sold by Boston Scientific Corporation is an example of a knitted Nitinol stent.
Another type of self-expanding stent that reverts to its original shape due to an elastic deformation when radially compressed is exemplified in U.S. Pat. No. 4,655,771, issued to Wallsten and incorporated herein by reference. Walisten discloses a self-expanding, braided surgical dilator stent particularly adapted for coronary dilation, but which can be adapted for use in other body vessels. That patent discloses a stent generally in accordance with the stent 10 shown in FIG. 1A. It comprises a hollow tubular member, the wall of which is formed of a series of individual, flexible, thread elements 12 and 14, each of which extends helically around the central longitudinal axis of the stent. A first subset of the flexible thread elements 12 have the same direction of winding and are displaced relative to each other about the cylindrical surface of the stent. They cross a second plurality of helical thread elements 14 which are also displaced relative to each other about the cylindrical surface of the stent, but having the opposite direction of winding. Accordingly, as shown in FIG. 1A, the threads 12 of the first subset cross the threads 14 of the second subset at crossing points 16.
As the stent is axially stretched, i.e., as the longitudinal ends 18 and 20 are forced away from each other, the diameter reduces, as shown in FIG. 1B. Likewise, if the wall of the stent is radially constricted so as to reduce the stent's diameter, the stent elongates. In other words, radial constriction and axial elongation go hand in hand. When the force is released, the stent tends to spring back to its resting diameter and length.
Bioabsorbable stents also are known in the prior art. Bioabsorbable stents are manufactured from materials that dissolve over an extended period of time when exposed to bodily fluids and are absorbed into the surrounding cells of the body. Various bioabsorbable materials that are suitable for fabricating stents are known in the prior art, including polymers such as poly-L,D-lactic acid, poly-L-lactic acid, poly-D-lactic acid, polyglycolic acid, polylactic acid, polycaprolactone, polydioxanone, poly(lactic acid-ethylene oxide) copolymers, or combinations thereof. Vainionp et al., Prog Polym. Sci., vol. 14, pp. 697-716 (1989); U.S. Pat. No. 4,700,704, U.S. Pat. No. 4,653,497, U.S. Pat. No. 4,649,921, U.S. Pat. No. 4,599,945, U.S. Pat. No. 4,532,928, U.S. Pat. No. 4,605,730, U.S. Pat. No. 4,441,496, and U.S. Pat. No. 4,435,590, all of which are incorporated herein by reference, disclose various compounds from which bioabsorbable stents can be fabricated.
Most, if not all, stents, need to be radially constricted, i.e., reduced in diameter, so that they can be inserted into the body lumen. Then, once they are in situ, the stent can be released and radially expanded.
Various delivery apparatus for delivering a stent into a body lumen in a radially constricted state and then releasing the stent so that it self expands within the body lumen are known. In one popular design illustrated for instance by the device disclosed in U.S. Pat. No. 5,026,377 and shown in FIG. 2, the delivery apparatus comprises an inner tube 5 surrounded by a concentric outer tube 1. The outer tube is shorter than the inner tube so that the inner tube can extend from the outer tube at both ends. A handle 6 typically is provided at the proximal end of the inner tube. Another handle 2 is provided at the proximal end of the outer tube. The inner core is slidable within the outer tube by relative manipulation of the two handles. A stent 11 is loaded within the delivery apparatus captured between the inner and the outer tubes near the distal end of the delivery apparatus.
The inner core may be hollow and adapted to accept a guide-wire 8 which, as is well known in the related arts, can be used to help guide the device to the stent delivery site in the body lumen 4.
During stent delivery, a physician typically will make an incision in the body lumen 4 at a location remote from the stent desired deployment site and then guide the stent delivery device into the body lumen until the distal end of the stent delivery device is at the stent deployment site. The outer tube 1 is then withdrawn proximally while the inner tube 5 is held stationary. Accordingly, the outer tube 1 slides over the stent 11, thus releasing it from radial constriction, whereby the stent radially expands and contacts the wall of the body lumen 4. The stent 11 is held in place by the frictional force between the lumen wall and the stent body resulting from the radial expansion force of the stent. The stent is now fully deployed and the delivery device can be retracted and the procedure concluded.
It is an object of the present invention to provide an improved method and apparatus for delivering a stent into a body lumen.